=Fam. 3. Cymothoidae.=[101]—This is a group of parasites more completely parasitic than the foregoing, but their outer organisation does not differ greatly from an ordinary Isopodan form. A great many very similar species are known which infest the gill-chambers, mouths, and skin of various fishes. The chief interest that attaches to them is found in the fact that a number of them, and perhaps all, are hermaphrodite, each individual acting as a male when free-swimming and young, and then subsequently settling down and becoming female. This condition is exactly the same as that occurring universally in the great group of parasitic Isopoda, the Epicarida, to be considered later. There is no evidence that the Cymothoidae are phyletically related to the Epicarida, so that the similar sexual organisation appears to be due to convergence resulting from similar conditions of life. The general question of hermaphroditism in the Crustacea has been shortly discussed on pp. 105–106. _Cymothoa._ =Fam. 4. Cirolanidae.=—In this family is placed the largest Isopod known—the deep-sea _Bathynomus giganteus_, found in the Gulf of Mexico and the Indian Ocean, sometimes measuring a foot long by four inches broad. A common small littoral form is _Cirolana_. =Fam. 5. Serolidae.=[102]—The genus _Serolis_ comprises flattened forms bearing a curious resemblance to Trilobites, which Milne Edwards considered more than superficial. The genus is confined to the littoral and deep waters of the southern hemisphere. =Fam. 6. Sphaeromidae.=[103]—These are flattened, broad-bodied forms, most commonly met with in the Mediterranean and warmer seas. Without being actually parasitic, they are frequently found as scavengers in decaying material, and they show some relationship to the parasitic Cymothoidae. In some of the genera, _e.g._ _Cymodoce_, the ovigerous female shows a degenerate condition of the mouth-parts, while the maxillipedes undergo an enlargement, and are used for causing a current through the brood-chamber. =Sub-Order 3. Valvifera.= [Illustration: FIG. 83.—_Munnopsis typica_ (Munnopsidae), ♂, × 2. _A_, 2nd antenna; _Ab_, abdomen; _T_, 5th thoracic appendage or 4th leg. (After Sars.) ] The Valvifera, illustrated by the =Idotheidae= and =Arcturidae=, are characterised by the uropods being turned back and expanded to form folding doors covering up the delicate pleopods, which are mostly respiratory in function, though the anterior pairs may serve as swimming organs. _Arcturus_ is a typically deep sea genus, many species, remarkably furnished with spiny processes, having been taken by the _Challenger_ in the southern hemisphere. The Idotheidae are more littoral forms, several species of _Idothea_ being commonly met with off the British coasts, occasionally penetrating into brackish or even fresh water. =Sub-Order 4. Asellota.=